The invention generally relates to the field of control systems, and more particularly to the field of process controller apparatus, in which a self-tuning variable setpoint weighting function is introduced to speed up the transient setpoint response without detrimental effect on load disturbance response characteristics.
For purposes of illustration, a PID (proportional-integral-derivative) controller apparatus is used to describe the prior art and an application of the invention, although the inventive variable setpoint weighting function is not limited thereto. The concepts of the invention are applicable to other controller apparatus such as PI, state variable feedback, Smith Predictor, etc. The description of the invention as applied to a PID controller is illustrative only and is not limiting on the scope of the invention.
In known controller systems the PID controller parameters are normally tuned to provide a desired load disturbance characteristic. When the dead time (.theta..sub.d) of the process is small relative to the dominant process the constant .tau. and a low value of normalized dead time .theta. results (wherein the normalized dead time is defined as a ratio .theta..sub.d /.tau.), it is found that the setpoint response exhibits a large overshoot. A conventional prior art solution is to detune the controller settings (in which case the load disturbance response characteristic becomes sluggish), or to introduce setpoint filtering--an approach which is termed two degrees of freedom by some instrument suppliers. Recently, use of a constant setpoint weighting factor .beta. has been introduced in the literature. Such a constant setpoint weighting factor is shown in the implementation of FIG. 1, wherein the process control is described by Equation (1): EQU U.sub.p =K.sub.c (.beta.Y.sub.r -Y) (1)
In Eq. (1), U.sub.p is the proportional part of the controller output, K.sub.c is the proportional gain, Y.sub.r is the setpoint and Y is the process variable. In conventional PID control, the setpoint weighting factor is set at a constant value 1. That is, in the prior art, .beta.=1. For constant set point weighting factor values which are smaller, i.e., for controllers wherein .beta.&lt;1, it is found that the large overshoot in setpoint response can be drastically reduced without having to sacrifice the load disturbance response.
FIGS. 2a and 2b show that setpoint weighting is superior to setpoint filtering and gain detuning, respectively, in terms of response speed. In FIG. 2a the setpoint response for a weighting factor .beta.=0.45, shown in curve (i), is seen to be faster than that for set point filtering shown in curve (ii). Similarly, in FIG. 2b the setpoint response for .beta.=0.45 (curve i) is faster than that obtained by detuning the gain (curve ii), and moreover the detuned gain curve shows a much poorer load disturbance response.
Thus, the method of setpoint filtering suffers from a major disadvantage in that the setpoint response speed (in terms of rise time) is sacrificed. However, provision of a constant (low) setpoint weighting factor suffers from a similar disadvantage when compared with the unity setpoint weighting factor. This can be clearly seen in FIG. 6, in which the response with setpoint weighting (.beta.=0.5 or 0.0) is much slower than that with setpoint weighting (.beta.=1), although a smaller overshoot is achieved thereby.
This problem cannot be resolved by current self-tuning controllers (e.g., U.S. Pat. No. 4,602,326, issued Jul. 22, 1986 to Kraus), which do not have any setpoint weighting function and in which the overshoot in setpoint response will be large after the controller is tuned for good load disturbance characteristics. Therein, if the controller is tuned using setpoint response, the load disturbance will be sluggish.